Automotive bodies and household electrical appliances and the like are produced by generating molded metal items from metal materials such as steel sheets, galvanized steel sheets and aluminum alloys, and then coating and assembling these items. Coating of these molded metal items is conducted via a number of steps including degreasing, surface conditioning, chemical conversion treatment, and electrodeposition coating.
The surface conditioning treatment is conducted to ensure that, in the subsequent phosphate coating chemical conversion treatment, a coating comprising phosphate crystals can be formed uniformly, rapidly, and with a high density across the entire surface of the metal, and the treatment usually comprises immersing the metal item in a surface conditioning bath in order to form phosphate crystal nuclei on the metal surface.
For example, Patent Reference 1 discloses a pretreatment liquid for conditioning a surface prior to a phosphate coating chemical conversion treatment, the liquid comprising at least one material selected from amongst phosphates containing at least one divalent or trivalent metal and including particles with a particle size of not more than 5 μm, an alkali metal salt or ammonium salt or a mixture thereof, and at least one material selected from the group consisting of anionically charged and dispersed fine oxide particles, anionic water-soluble organic polymers, nonionic water-soluble organic polymers, anionic surfactants and nonionic surfactants, and having a pH within a range from 4 to 13.
Furthermore, Patent Reference 2 discloses a pretreatment liquid for conditioning a surface prior to phosphate coating chemical conversion treatment, the liquid comprising one or more types of phosphate particles selected from amongst phosphates containing one or more divalent and/or trivalent metals, and further comprising (1) one or more materials selected from amongst monosaccharides, polysaccharides, and derivatives thereof, (2) one or more materials selected from amongst orthophosphoric acid, polyphosphoric acid or organic phosphonic acid compounds, and water-soluble polymer compounds comprising a vinyl acetate polymer, a derivative thereof, or a copolymer of vinyl acetate and another monomer that is copolymerizable with vinyl acetate, or (3) a polymer or copolymer obtained by polymerizing at least one compound selected from amongst specific monomers or α,β-unsaturated carboxylic acid monomers, and not more than 50% by weight of another monomer that is copolymerizable with the above monomer.
Moreover, Patent Reference 3 discloses a method for surface treatment of an aluminum alloy malleable material, wherein the zeta potential is regulated.
However, when the surface conditioning treatment liquids disclosed in the Patent Reference 1 and the Patent Reference 2 are used, rust can sometimes develop following the surface conditioning, meaning further improvements are desirable.
Furthermore, with the surface conditioning treatment liquids disclosed in the Patent Reference 1 and the Patent Reference 2, in those areas where an aluminum alloy contacts a steel sheet or a galvanized steel sheet, the aluminum alloy portion becomes an anode and the steel sheet or galvanized steel sheet becomes a cathode, making formation of a chemical conversion coating on the aluminum alloy difficult. As a result, there is a demand for the development of a surface conditioner that is capable of suppressing electrolytic corrosion on an aluminum alloy during chemical conversion treatment.
Moreover, when the surface conditioning treatment liquids described above are used with aluminum alloys or metals such as high-tensile steel sheets, a satisfactory chemical conversion coating is not formed on the surface of the metal. Furthermore, when these surface conditioning treatment liquids are used, other problems also arise, including an inability to form a dense chemical conversion coating on a variety of metal materials, and the fact that because the particle size of the particles within the surface conditioning treatment liquids is large, the stability of the particles within the treatment bath tends to be unsatisfactory, causing ready precipitation of the particles.
Furthermore, even when surface conditioning using the treatment liquid disclosed in the Patent Reference 3 is conducted, it is unable to satisfactorily resolve the types of problems outlined above.    [Patent Reference 1] Japanese Patent Laid-Open No. H10-245685    [Patent Reference 2] Japanese Patent Laid-Open No. 2000-96256    [Patent Reference 3] Japanese Patent Laid-Open No. 2001-262364